1. Field of the Invention
This invention relates to uninterruptible power supplies, and particularly to the control of active rectifiers for uninterruptible power supplies.
2. Description of Background
Batteries that are implemented within, uninterruptible power supply (UPS) systems typically surfer from ripple currents and fast discharge pulses that can affect the life of a system-linked battery. Under unbalanced load conditions, an output inverter draws at twice the mains frequency from a direct current (dc) Sink and an oscillating current that is produced by a rectifier and the battery. Normal rectifiers can only provide the mean value of this oscillating current, therefore, the ripple current must be drawn from the battery, thus affecting the life of the battery. In the event that a UPS load is unbalanced then, the associated active power is not constant. In this instance there is the possibility for two extreme operating conditions to exist. The first operating condition requires the drawing of constant active power from a utility, thus resulting in the acquisition of sinusoidal currents at the price of producing a high current ripple on the battery. The second operating condition requires the drawing of oscillating active power from the utility, and therefore acquiring non-sinusoidal currents, wherein the operation is ideally implemented without drawing any battery current.
Conventionally, six or twelve pulse thyristor rectifiers, sometimes in combination with passive filters, have been used as inputs for an uninterruptible power supply (UPS). However, using these particular topologies, it was not possible to obtain the fast current control that is required to solve the above-mentioned problems. Currently, more modern power topologies are utilized to realize clean input UPSs that can draw sinusoidal currents from the mains at a high power factor. Typical front-end converters include current or voltage source rectifiers, these often being referred to as IGBT rectifiers. These converters make it possible to reach a required bandwidth for instituting a current control operation. However, additional control effort is required to take advantage of this fast current control in order to solve the mentioned problems, i.e. increase the battery life and obtain a very stiff dc link voltage control.